The prior art is replete with suggestions for methods for preparing articles which comprise thermosetting resins and inorganic particulate materials. Many techniques have been developed for controlling the physical properties of such articles depending on the anticipated end-use. Molded articles comprising resins and inorganic particulates have been utilized as abrasive articles such as grinding wheels, segments and discs, polishing or finishing tools which may be in the shape of wheels and discs, and honing tools.
Articles comprising resins and inorganic particulates also may be formed in shapes which are suitable for use as molds for casting various materials including plastic and metal castings. Such articles also can be prepared containing controlled and graded pore sizes, and such articles are useful as filter elements.
Accordingly, although the emphasis of this disclosure, particularly from the standpoint of best modes presently known to the inventor, is on abrasive articles, there is no intention or reason to limit the specific end use of the novel articles of this invention.
Abrasive articles such as grinding wheels are commonly produced by ramming discrete abrasive particles or grit in a suitable mold together with a ceramic material which is subsequently fused at a very high temperature to bond such particles together. These generally are referred to as ceramic or vitreous grinding wheels. Grinding wheels so produced are often fragile, out of balance and non-uniform in density of abrasive grain. When resin bonding agents have been employed in the formation of grinding wheels (generally referred to as resinoid wheels), many of the same problems have been encountered, and the bond to the abrasive grains has been imperfect and flawed, with voids caused by entrapped air and absorbed moisture, and physical stresses in the cured resin binder resulting in weaker retention of the grains under conditions of use.
Foamed resins also have been employed in the preparation of abrasive articles which evince certain practical advantages, but nevertheless have the unavoidable concommitant of reducing the abrasive grain density and hence the number of cutting points at the working face of the tool. As abrasive machining has grown in industrial importance, the demand has increased for abrasive tools, and especially grinding wheels, which are capable of extremely rapid cutting action while still maintaining a true tool profile to ensure dimensionally accurate geometry of the work surface.
One method known in the prior art for obtaining the desired high concentration of abrasive grains in the grinding articles involves mixing the grains and the binder, and forming the article under extremely high pressure. This method, though permitting the formation of articles containing a high concentration of grain, does not provide any control of the grain arrangement nor particularly with respect to the uniformity of the dispersion of the grains in the binder. In other prior art methods, mixtures of grain and resin are centrifuged to concentrate the grain at the periphery of the mold with only a minimal amount of resin in this area for bonding. The grains in such prepared wheels are in initimate contact with one another often resulting in a build-up of stress concentration and heat on grinding which weakens the wheel and causes metallurgical damage. Examples of prior art patents describing foaming procedures for preparing abrasive tools include U.S. Pat. Nos. 2,885,276; 3,377,411 and 3,850,589.
Nestor, U.S. Pat. No. 2,862,806 has described a procedure wherein abrasive particulates are added in dry condition to the mold and the mold is rotated at about 1750 rpm while the resin binder, heated to about 120.degree. C. is mixed with hardener and poured through a central access into the mold and onto the abrasive at room temperature. While continuing the centrifuging, the mold is heated causing the contents of the mold to be heated to thermoset and shrink the replicate mass in the mold so that removal from the mold is simplified. The articles prepared in this matter are observed to contain variable abrasive/bond ratios from the central core continuously radially outwardly. Little control of heating or of the ratio of abrasive particulate to bond appears to be practical with the method.
The application of heat to the constituents utilized in the formation of grinding wheels, either individually or in combination at various stages of the processes have been suggested in the prior art. For example, Upton in U.S. Pat. No. 2,885,276 suggests that wetting of the abrasive grain by the resin can be improved when the resin is in the form of a continuous homogeneous liquid of low viscosity and surface tension. The patentee suggests the use of heated molds under certain conditions and certain applications as well as control of the temperature of the mineral grain and base resin in order to facilitate handling of the reactive mix from the mixing vessel to the mold. The patentee also suggests that control of the base resin temperature is a factor which aids in the control of viscosity and of uniformity of wetting of the abrasive grains. In column 5, lines 6-34, the desirability of controlling the temperature of the abrasive or polishing agent before introduction into the resin mix is discussed. Temperature control of the various ingredients is described as being critical to efficient foaming of the composition as the desired abrasive product is prepared.
In U.S. Pat. No. 3,850,589, there is a suggestion in column 12 beginning at line 19 that the resin component is normally heated to a suitable temperature so that the resin can be pumped and dispensed more readily. In column 12 of the same patent, beginning at line 26, there is a suggestion that the ingredients may be mixed at room temperature or may be preheated to reduce viscosity and increase the rate of reaction. A similar disclosure is contained in U.S. Pat. No. 3,377,411. Although the prior art foamed abrasive articles have exhibited good working characteristics, such grinding articles have not been suitable for all applications, and have not been the final answer to the problems of the grinding industry, probably because of the presence of the voids, channels and other discontinuities resulting from the foaming process. Such discontinuities, when contained in the resin bond, must be considered as flaws in the cured article.
Accordingly, there continues to be a need in the industry to provide molded articles, and particularly abrasive articles, having the desired (generally high) grain density at the working face with such grains being exceptionally securely held by a substantially unflawed resin bond and yet allowing interstices to be present in amounts compatible with the function the article is to perform. Such a condition is not met by the utilization of the general knowledge that heat improves the wetting process, especially when thermosetting plastics are also very sensitive to drastic reduction in pot life when so conditioned. The concept of this invention is self-defeating until it is carried to an extreme where unexpectedly the very low viscosity of the bond and the super-receptivity of the abrasive combines to unexpectedly shorten mixing time so that shortened pot life loses its importance. Such references in the art as that heat can be utilized to improve wetting do not reach or even remotely suggest the new concept of this invention where the two phase approach calls for two levels of control--once to achieve supermixing and wetting in spite of pot life considerations and second to ameliorate this first unstable condition by controlling the temperature via heat sink conditions and subsequent heat additions to allow a normal polymerization of the bond. Even the results obtained are unexpected. With this new process it is possible to make a full range of products from extremely low bond ratio to high bond ratio products where complete wetting between the bond and the abrasive occurs, with the bond films relatively free from even small cells, air spaces, and voids which weaken such structural members, and with such a percentage of interstices as are considered desirably sufficient even to make filters. The differences between interstices and void space, frothiness, cells or other defects in the bond films being suddenly recognized as counterproductive in the manufacture of such a product as grinding wheels where high strength (safety) must be balanced with the conditions of the grinding work to be done. Prior art techniques also have been deficient in permitting the preparation of a broad spectrum of grinding articles containing, when desired, abrasive to bond ratios ranging from about 1:1 to about 150:1.